Why do they do it? What convinces famous people to take the cursed fifth seat on Question Time? As proved by the ordeal suffered by such unfortunates as Rhona Cameron, Irvine Welsh and Will Young (much worse than subsequent accounts made out), the gig amounts to a dependable trap door, through which composure and articulacy will tumble, more often than not.

It is presumably petrifying, and the appearance money can't be very much. Matters of ego may provide part of the answer, though some people claim it's actually got more to do with being public spirited. When I asked Blur's Alex James – who did not have the most comfortable QT experience – why he did it, he said this: "It's terrifying. But it's like doing jury service: if you get the call, you should go. I'm glad I did it, even if it was the wrong thing to do."

And so to Jarvis Cocker, who last night took his place next to Harriet Harman, a very unhinged Iain Duncan Smith, the Liberal Democrat David Laws, and that rightwing populist psychonaut Peter Hitchens. Cocker was booked, I dare say, thanks to the current domination of world events by Michael Jackson, thought in a bamboozling example of the BBC somehow forgetting itself, that subject didn't come up until two minutes from the end – which left poor old Jarvis floundering.

The programme came from groovy(ish) Cambridge, and the audience were palpably on his side. In the right place, he can undoubtedly turn on the wit, and his best lyrics (while we're here, a brief mention for Weeds, Cocaine Socialism and Help the Aged), surely suggest an erudite observer of modern life. By and large, however, nothing happened: he had apparently not done the no-brainer preparation of familiarising himself with the week's big stories, nor thought of any great lines in advance. He looked lost, really; a frontman reduced to being a spectator – even, dare I say it, the drummer to the odious Hitchens's lead vocalist.

This, for example, was his initial answer to a question about Gordon Brown's alleged dishonesty about the inevitability of public spending cuts: "I can't pretend to be particularly informed on this subject. It seems strange to be getting het-up about it." There followed an undeveloped point about why the public sector deserved as much money as the banks, and a cop-out bit of anti-politics about everything amounting to "my gang versus your gang". There was something to his views on Ronnie Biggs – essentially, a plea for some compassion – but his understanding of the Great Train Robbery was fuzzy indeed.

He was mildly funny about five-year licences for teachers, but it may well have been the first he'd heard about it. And hats off for demanding the renationalisation of the railways, but he didn't really have an argument. Even on Jackson, there wasn't much in the way of an inspired answer. With the seconds ticking away, David Dimbleby asked him whether Jacko had been a genius. "Yes," said Cocker. "He invented the moonwalk." Boom boom.

I'm not enjoying writing this, incidentally, but anyway: what was most disappointing was a behavioural tic I've observed in scores of musicians: their tendency to respond to big events and ideas with not much more than sighing bemusement. Judging by some of Cocker's songs, there is rage in his soul – or there used to be, at least – but none of it came out last night. The most disappointing moment was when Hitchens once again reprised his back-to-the-50s shtick, and demanded the return of grammar schools, and Cocker said nothing at all. The remaining members of his old charity-shop army were presumably as underwhelmed as everyone else; like so many previous QT trap-door victims, I wouldn't imagine he'll be back.